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We have synthesized a series of nanocrystalline ferrite samples with the composition MnxCu1-xFe2O4 (x=0.2, 0.4, 0.6, 0.8) by an advanced sol-gel auto-combustion method. The X-ray diffraction patterns confirm the existence of single-phase cubic spinel crystal structure of ferrites with lattice parameter ranges from 8.395 ?? to 8.45 ??. We report the equilibrium radii for tetrahedral and octahedral sites in the unit cells and the estimated cation distribution over the two sites of nanocrystalline MnxCu1-xFe2O4. We also estimate the oxygen positional parameter as 0.389. The positron annihilation life time spectroscopic studies were carried out for all the samples and analyzed the variation of life time parameters ??1; I1, ??2; I2 and the mean life time ??m to elucidate the defect structure of the nanocrystalline MnxCu1-xFe2O4. We found that the overall vacant type defects fill up as the Mn2+ ion concentration, x, increases. The value of ??1 varies from 150 ps to 170 ps and that of ??2 varies from 295 ps to 335 ps, which are the characteristic values for nanocrystalline samples, indicating the presence of intergranular as well as surface-diffused vacancies in the crystal structure.
Date of Publication: March 2010